In fill level measurement, microwaves are transmitted via an antenna to the surface of a fill substance and the echo waves reflected on the surface are received. An echo-function representing echo amplitudes as a function of distance is formed, from which the probable useful echo and its travel time are determined. The distance between the surface of the fill substance and the antenna is determined from the travel time.
All known methods that make it possible to measure relatively short distances by means of reflected microwaves can be applied. The best known examples are pulse radar and frequency modulation continuous wave radar (FMCW-Radar).
With pulse radar, short microwave send pulses, referenced in the following as wave packets, are periodically transmitted, reflected from the fill substance surface and received after a distance-dependent travel time. The received signal amplitude as a function of time represents the echo function. Each value of this echo-function corresponds to the amplitude of an echo reflected at a specific distance from the antenna.
With the FMCW method, a continuous microwave is sent that is periodically linearly frequency-modulated, for example according to a sawtooth wave function. The frequency of the received echo signal, therefore, has a frequency difference compared to the instantaneous frequency which the send signal has at the time of receipt. This frequency difference is a function of the travel time of the echo signal. The frequency difference between send signal and received signal, which can be determined by mixing both signals and evaluating the Fourier spectrum of the mixed signals, thus corresponds to the distance of the reflecting surface from the antenna. In addition, the amplitudes of the spectral lines determined through Fourier transformation correspond to the echo amplitudes. This Fourier spectrum, therefore, represents in this case the echo-function.
Fill level measuring devices working with microwaves are deployed in many branches of industry, e.g. in the chemicals industry and in the foods industry.
There are many applications in which fill level devices are exposed to extreme measuring conditions, e.g. high temperatures, high pressures and/or chemically aggressive substances. Fill level measuring devices working with microwaves have sensitive components, e.g. measuring instrument electronics and a sending and/or receiving element for microwaves, that cannot withstand such extreme conditions.
This problem and a solution therefor are described in EP-A 0 943 902. A microwave-based fill level measuring device with antenna is described there,                which has a cylindrical segment closed at one end,        in which a microwaves sending and/or receiving element is arranged,        which has a radiating and/or intake element,        which has functional elements between the segment and the radiating and/or intake element.        
One functional element described, among others, is a spacer, which serves to increase the separation between temperature sensitive components of the fill level measuring device and the radiating and/or intake element. A process separation is also described.
These functional elements protect the sensitive components of the microwave-based fill level measuring device against the circumstances of extreme measuring conditions. It is thus possible to deploy these fill level measuring devices in a multiplicity of very different applications.
A large number of different versions of fill level measuring devices working with microwaves are charged with serving this multiplicity of applications. Thus, for example, under simple measuring conditions, a fill level measuring device is used in which the segment which contains the sending and/or receiving element is directly connected to the radiating and/or intake element. If, in contrast, more extreme measuring conditions prevail, then corresponding functional elements are necessary for protecting the sensitive components of the fill level measuring device. For example, if high temperatures prevail at the location of measurement, then a spacer of adequate length is to be provided. If high pressures prevail, a sufficiently pressure resistant process separation is to be provided. A chemically highly resistant process separation is to be provided, in the case of chemically aggressive media.
This means a large variety of devices, which requires an accordingly large inventory for the manufacturer and the user.